The tomato leaf miner Tuta absoluta (Meyrick) may pose a threat to both greenhouse and open-field tomato production. The aims of this study were: 1) to assess the susceptibility to deltamethrin of seven field populations of T. absoluta in Hamedan province, in comparison with susceptible strain. 2) to investigate the effects of synergists for testing possible mechanisms involved in resistance. Bioassays were done by leaf dipping method to determine the resistance level. Resistance ratios calculated varied from 1.9- to 21.6-times (compared with the susceptible population). In synergism tests, PBO, DEF, and DEM were used to determine whether metabolism was involved in deltamethrin resistance. Tomato leaves treated with appropriate concentration of each synergist were fed to second instar larvae for 12 h. The concentrations of PBO, DEF, and DEM were 100, 100 and 120 mgL-1, respectively. These were the highest concentrations that caused no mortality in susceptible strain in preliminary tests. The cytochrome P450-dependent monooxygenase- inhibitor PBO significantly synergized the activity of deltamethrin in the three greenhouse field populations (with the highest synergism ratio of 5.4). No significant synergism of deltamethrin toxicity was observed when larvae were pretreated with the esterase-inhibitor DEF, and the glutathione depleter DEM, as indicated by the overlap in the 95% CL for treatment with deltamethrin alone or with deltamethrin +DEF/ DEM.